1. Field of the Invention
The invention relates to a capacitor; in particular, to a capacitance measuring circuit and a capacitance measuring method.
2. Description of the Prior Art
Since capacitive touch panels and capacitive fingerprint identification devices have been widely used in today's electronic products, the technologies of detecting capacitance variations have become more and more important. In the prior arts, such as patents of TW 1361280 and U.S. Pat. No. 8,134,374 (as shown in FIG. 1A and FIG. 1B respectively), in order to remove most of the basic capacitance and keep most of the detection range of the detection module to detect the real capacitance variations, the capacitance compensation circuit is usually used to cancel off most of the charges induced by the basic capacitance.
For example, Cx in FIG. 1A represents the basic capacitance of the capacitor to be detected. In order to cancel off most of the charges induced by the basic capacitance, the switch SW2 in the capacitance compensation circuit is switched to make the voltage variation at one terminal of the compensation capacitor Cadj opposite to the voltage variation at one terminal of the capacitor Cx to be detected and the control signal Sadj is used to adjust the compensation capacitor Cadj, so that most of the charges at the output terminal ndo and the input terminal ndi of the integrator circuit induced by the capacitor Cx to be detected will be cancelled off by the charges with opposite polarization induced by the compensation capacitor Cadj. As shown in FIG. 1A, when the clock signal CK_a1 is 1, one terminal nd2 of the compensation capacitor Cadj is charged to the voltage level Vf2 and one terminal nd1 of the capacitor Cx to be detected is discharged to the voltage level Vf1; when the clock signal CK_a2 is 1, the positive charges of the compensation capacitor Cadj will be cancelled off by the negative charges of the capacitor Cb2 to be detected; therefore, most of the basic capacitance will be removed.
Since the parasitic capacitance of the in-cell capacitive touch panel or the large-sized capacitive touch panel is large, e.g., in a range of 100 pF˜1000 pF, when the capacitance of the in-cell capacitive touch panel or the large-sized capacitive touch panel is detected, the parasitic capacitance of the touch panel not touched will become the basic capacitance (e.g., Cx in FIG. 1A). Since the capacitive touch apparatus detects the capacitance variations before and after the touch panel is touched and determines whether the touch panel is touched and the touched positions on the touch panel, if the capacitive touch apparatus is applied to the in-cell capacitive touch panel or the large-sized capacitive touch panel, there will be large basic capacitance to be cancelled off and the capacitance of the compensation capacitor (e.g., Cadj in FIG. 1A) needed will be also larger. Therefore, the volume and the cost of the detection apparatus will be increased. If the detection apparatus is an integrated circuit, the area of the compensation capacitor in the integrated circuit will be increased and the cost will be also increased.